This invention relates to a method and apparatus for measuring the rotation of a work table and, more particularly, to the production of a digital readout of the position of such a table.
Resolvers are well known devices for measuring the linear or rotary displacement of a member. The principles of these devices are explained, for example, in U.S. Pat. No. 2,799,835 to Tripp et al., U.S. Pat. No. 2,867,783 to Childs and U.S. Pat. No. 2,915,721 to Farrand et al., which are incorporated herein by reference. The basic circuit arrangement for rotary measurements consists of a set of quadrature windings which are spaced 90.degree. from each other, and a sensor winding which rotates in close proximity to the quadrature windings. The sensor winding is attached to the member whose angular displacement .theta.' is to be measured. If currents in the relation of sine .phi. and cosine .phi. are supplied to the quadrature windings, there will be a particular position of the sensor winding where the current induced therein from the quadrature windings is zero, i.e. when .theta.=.phi.. In all other positions e.g. .theta.', an error current will be detected which can be measured or used in a feedback circuit to vary the sine and cosine inputs until a null is achieved, i.e. .phi. is changed until it equals .theta.. The change needed to vary the sine and cosine functions is an indication of the rotation from the position .theta. to the position .theta.'.
The sine and cosine function can be generated by applying the output of a signal generator to two multiplying digital to analog converter (MDAC) units. The multiplying factor for the MDAC units is determined in response to the error signal such that one unit produces a sine function and the other produces a cosine function of the angular position where a null is achieved.
In order to improve the accuracy of the resolver, multiple poles can be used instead of a single pole. As a result, a motion from one pole position to another will represent only a small part of one revolution of the mechanical member being rotated. Such multi-pole resolvers must be manufactured with a high degree of accuracy in order to produce accurate measurements. Also the sine and cosine MDAC units must accurately produce sine and cosine functions. As a result, prior art multi-pole resolvers are expensive and difficult to manufacture.